1. Field of the Invention
The present invention relates to a cover for a golf ball. More specifically, the present invention relates to a golf ball cover layer composed of a polyurethane formed from a blend of diisocyanate prepolymers.
2. Description of the Related Art
Conventionally golf balls are made by molding a cover around a core. The core may be wound or solid. A wound core typically comprises elastic thread wound about a solid or liquid center. Unlike wound cores, solid cores do not include a wound elastic thread layer. Solid cores typically may comprise a single solid piece center or a solid center covered by one or more mantle or boundary layers of material.
The cover may be injection molded, compression molded, or cast over the core. Injection molding typically requires a mold having at least one pair of mold cavities, e.g., a first mold cavity and a second mold cavity, which mate to form a spherical recess. In addition, a mold may include more than one mold cavity pair.
In one exemplary injection molding process each mold cavity may also include retractable positioning pins to hold the core in the spherical center of the mold cavity pair. Once the core is positioned in the first mold cavity, the respective second mold cavity is mated to the first to close the mold. A cover material is then injected into the closed mold. The positioning pins are retracted while the cover material is flowable to allow the material to fill in any holes caused by the pins. When the material is at least partially cured, the covered core is removed from the mold.
As with injection molding, compression molds typically include multiple pairs of mold cavities, each pair comprising first and second mold cavities that mate to form a spherical recess. In one exemplary compression molding process, a cover material is pre-formed into half-shells, which are placed into a respective pair of compression mold cavities. The core is placed between the cover material half-shells and the mold is closed. The core and cover combination is then exposed to heat and pressure, which cause the cover half-shells to combine and form a full cover.
As with the above-referenced processes, a casting process also utilizes pairs of mold cavities. In a casting process, a cover material is introduced into a first mold cavity of each pair. Then, a core is held in position (e.g. by an overhanging vacuum or suction apparatus) to contact the cover material in what will be the spherical center of the mold cavity pair. Once the cover material is at least partially cured (e.g., a point where the core will not substantially move), the core is released, the cover material is introduced into a second mold cavity of each pair, and the mold is closed. The closed mold is then subjected to heat and pressure to cure the cover material thereby forming a cover on the core. With injection molding, compression molding, and casting, the molding cavities typically include a negative dimple pattern to impart a dimple pattern on the cover during the molding process.
Materials previously used as golf ball covers include balata (natural or synthetic), gutta-percha, ionomeric resins (e.g., DuPont's SURLYN.RTM.), and polyurethanes. Balata is the benchmark cover material with respect to sound (i.e. the sound made when the ball is hit by a golf club) and feel (i.e. the sensation imparted to the golfer when hitting the ball). Natural balata is derived from the Bully Gun tree, while synthetic balata is derived from a petroleum compound. Balata is expensive compared to other cover materials, and golf balls covered with balata tend to have poor durability (i.e. poor cut and shear resistance). Gutta percha is derived from the Malaysian sapodilla tree. A golf ball covered with gutta percha is considered to have a harsh sound and feel as compared to balata covered golf balls.
Ionomeric resins, as compared to balata, are typically less expensive and tend to have good durability. However, golf balls having ionomeric resin covers typically have inferior sound and feel, especially as compared to balata covers.
A golf ball with a polyurethane cover generally has greater durability than a golf ball with a balata cover. The polyurethane covered golf ball generally has a better sound and feel than a golf ball with an ionomeric resin cover. Polyurethanes may be thermoset or thermoplastic. Polyurethanes are formed by reacting a prepolymer with a polyfunctional curing agent, such as a polyamine or a polyol. The polyurethane prepolymer is the reaction product of, for example, a diisocyanate and a polyol such as a polyether or a polyester. Several patents describe the use of polyurethanes in golf balls. However, golf balls with polyurethane covers usually do not have the distance of other golf balls such as those with covers composed of SURLYN.RTM. materials.
Gallagher, U.S. Pat. No. 3,034,791 discloses a polyurethane golf ball cover prepared from the reaction product of poly(tetramethylene ether) glycol and toluene-2,4-diisocyanates (TDI), either pure TDI or an isomeric mixture.
Isaac, U.S. Pat. No. 3,989,568 ("the '568 patent) discloses a polyurethane golf ball cover prepared from prepolymers and curing agents that have different rates of reaction so a partial cure can be made. The '568 patent explains that "the minimum number of reactants is three." Specifically, in '568 patent, two or more polyurethane prepolymers are reacted with at least one curing agent, or at least one polyurethane prepolymer is reacted with two or more curing agents as long as the curing agents have different rates of reaction. The '568 patent also explains that "[o]ne of the great advantages of polyurethane covers made in accordance with the instant invention is that they may be made very thin. . . . ", and "[t]here is no limitation on how thick the cover of the present invention may be but it is generally preferred . . . that the cover is no more than about 0.6 inches in thickness." The examples in the '568 patent only disclose golf balls having covers that are about 0.025 inches thick.
Dusbiber, U.S. Pat. No. 4,123,061 ("the '061 patent")discloses a polyurethane golf ball cover prepared from the reaction product of a polyether, a diisocyanate and a curing agent. The '061 patent discloses that the polyether may be polyalkylene ether glycol or polytetramethylene ether glycol. The '061 patent also discloses that the diisocyanate may be TDI, 4,4'-diphenylmethane diisocyanate ("MDI"), and 3,3'-dimethyl-4,4'-biphenylene diisocyanate ("TODI"). Additionally, the '061 patent discloses that the curing agent may be either a polyol (either tri- or tetra-functional and not di-functional) such as triisopropanol amine ("TIPA") or trimethoylol propane ("TMP"), or an amine-type having at least two reactive amine groups such as: 3,3' dichlorobenzidene; 3,3' dichloro 4,4' diamino diphenyl methane ("MOCA"); N,N,N',N' tetrakis (2-hydroxy propyl) ethylene diamine; or Uniroyal's Curalon L which is an aromatic diamine mixture.
Hewitt, et al., U.S. Pat. No. 4,248,432 ("the '432 patent") discloses a thermoplastic polyesterurethane golf ball cover formed from a reaction product of a polyester glycol (molecular weight of 800-1500) (aliphatic diol and an aliphatic dicarboxylic acid) with a para-phenylene diisocyanate ("PPDI") or cyclohexane diisocyanate in the substantial absence of curing or crosslinking agents. The '432 patent teaches against the use of chain extenders in making polyurethanes. The '432 patent states, "when small amounts of butanediol-1,4 are mixed with a polyester . . . the addition results in polyurethanes that do not have the desired balance of properties to provide good golf ball covers. Similarly, the use of curing or crosslinking agents is not desired. . . . "
Holloway, U.S. Pat. No. 4,349,657 ("the '657 patent") discloses a method for preparing polyester urethanes with PPDI by reacting a polyester (e.g. prepared from aliphatic glycols having 2-8 carbons reacted with aliphatic dicarboxylic acids having 4-10 carbons) with a molar excess of PPDI to obtain an isocyanate-terminated polyester urethane (in liquid form and stable at reaction temperatures), and then reacting the polyester urethane with additional polyester. The '657 patent claims that the benefit of this new process is the fact that a continuous commercial process is possible without stability problems. The '657 patent further describes a suitable use for the resultant material to be golf ball covers.
Wu, U.S. Pat. No. 5,334,673 ("the '673 patent") discloses a polyurethane prepolymer cured with a slow-reacting curing agent selected from slow-reacting polyamine curing agents and difunctional glycols (i.e., 3,5-dimethylthio-2,4-toluenediamine, 3,5-dimethylthio-2,6-toluenediamine, N,N'-dialkyldiamino diphenyl methane, trimethyleneglycol-di-p-aminobenzoate, polytetramethyleneoxide-di-p-aminobenzoate, 1,4-butanediol, 2,3-butanediol, 2,3-dimethyl-2,3-butanediol, ethylene glycol, and mixtures of the same). The polyurethane prepolymer in the '673 patent is disclosed as made from a polyol (e.g., polyether, polyester, or polylactone) and a diisocyanate such as MDI or TODI. The polyether polyols disclosed in the '673 patent are polytetramethylene ether glycol, poly(oxypropylene) glycol, and polybutadiene glycol. The polyester polyols disclosed in the '673 patent are polyethylene adipate glycol, polyethylene propylene adipate glycol, and polybutylene adipate glycol. The polylactone polyols disclosed in the '673 patent are diethylene glycol initiated caprolactone, 1,4-butanediol initiated caprolactone, trimethylol propane initiated caprolactone, and neopentyl glycol initiated caprolactone.
Cavallaro, et al., U.S. Pat. No. 5,688,191 discloses a golf ball having core, mantle layer and cover, wherein the mantle layer is either a vulcanized thermoplastic elastomer, functionalized styrene-butadiene elastomer, thermoplastic polyurethane, metallocene polymer or blends of the same and thermoset materials.
Wu, et al., U.S. Pat. No. 5,692,974 discloses golf balls having covers and cores that incorporate urethane ionomers (i.e. using an alkylating agent to introduce ionic interactions in the polyurethane and thereby produce cationic type ionomers).
Sullivan, et al., U.S. Pat. No. 5,803,831 ("the '831 patent") discloses a golf ball having a multi-layer cover wherein the inner cover layer has a hardness of at least 65 Shore D and the outer cover layer has a hardness of 55 Shore D or less, and more preferably 48 Shore D or less. The '831 patent explains that this dual layer construction provides a golf ball having soft feel and high spin on short shots, and good distance and average spin on long shots. The '831 patent provides that the inner cover layer can be made from high or low acid ionomers such as SURLYN.RTM., ESCOR.RTM. or IOTEK.RTM., or blends of the same, nonionomeric thermoplastic material such as metallocene catalyzed polyolefins or polyamides, polyamide/ionomer blends, polyphenylene ether/ionomer blends, etc., (having a Shore D hardness of at least 60 and a flex modulus of more than 30000 psi), thermoplastic or thermosetting polyurethanes, polyester elastomers (e.g. HYTREL.RTM.), or polyether block amides (e.g. PEBAX.RTM.), or blends of these materials. The '831 patent also provides that the outer cover layer can be made from soft low modulus (i.e. 1000-10000 psi) material such as low-acid ionomers, ionomeric blends, non-ionomeric thermoplastic or thermosetting materials such as polyolefins, polyurethane (e.g. thermoplastic polyurethanes like TEXIN.RTM., PELETHANE.RTM., and thermoset polyurethanes like those disclosed in Wu, U.S. Pat. No. 5,334,673), polyester elastomer (e.g. HYTREL.RTM.), or polyether block amide (e.g. PEBAX.RTM.), or a blend of these materials.
Hebert, et al., U.S. Pat. No. 5,885,172 ("the '172 patent") discloses a multilayer golf ball giving a "progressive performance" (i.e. different performance characteristics when struck with different clubs at different head speeds and loft angles) and having an outer cover layer formed of a thermoset material with a thickness of less than 0.05 inches and an inner cover layer formed of a high flexural modulus material. The '172 patent provides that the outer cover is made from polyurethane ionomers as described in Wu, et al., U.S. Pat. No. 5,692,974, or thermoset polyurethanes such as TDI or methylenebis-(4-cyclohexyl isocyanate) ("HMDI"), or a polyol cured with a polyamine (e.g. methylenedianiline (MDA)), or with a trifinctional glycol (e.g., N,N,N',N'-tetrakis(2-hydroxpropyl)ethylenediamine). The '172 also provides that the inner cover has a Shore D hardness of 65-80, a flexural modulus of at least about 65,000 psi, and a thickness of about 0.020-0.045 inches. Exemplary materials for the inner cover are ionomers, polyurethanes, polyetheresters (e.g. HYTREL.RTM.), polyetheramides (e.g., PEBAX.RTM.), polyesters, dynamically vulcanized elastomers, functionalized styrene-butadiene elastomer, metallocene polymer, blends of these materials, nylon or acrylonitrile-butadiene-styrene copolymer.
Wu, U.S. Pat. No. 5,484,870 ("the '870 patent") discloses golf balls having covers composed of a polyurea composition. The polyurea composition disclosed in the '870 patent is a reaction product of an organic isocyanate having at least two functional groups and an organic amine having at least two functional groups. One of the organic isocyanates disclosed by the '870 patent is PPDI.
Although the prior art has disclosed golf ball covers composed of many different materials, none of these golf balls have proven completely satisfactory. Dissatisfaction, for example, remains with processing and manufacturing the balls, and with the balls' durability and performance.
Specifically, with respect to processing, prior materials are not user friendly because certain starting materials may be unhealthful, such as diamines and isocyanides. In addition, prior balls using such materials are generally wound balls. Wound balls have tolerances that are more difficult to control due to core sizes and/or windings sizes, and therefore, require thicker cover layers to account for the manufacturing tolerances. With respect to durability problems, prior polyurethane covered balls, because they are wound balls, tend to lose compression and initial velocity due to the windings relaxing over time and use. With respect to performance problems, prior balls, as a general rule, tend to have smaller cores that result in shorter flight distances. Although many golf balls having a polyurethane cover have been provided by the prior art, these golf balls have failed to capture the sound and feel of balata while providing a golf ball with the durability of an ionomer.